Concerns about global warming have led to a worldwide effort in the science community to reduce greenhouse gas emissions and to increase energy efficiency. Conservation of energy through gradual diversion of resources to renewable green energy paves the way for tomorrow's consumption and a cleaner environment. Solid-state lighting (“SSL”) technology in the form of light-emitting diodes (“LEDs”) generates high-efficiency light sources, converts electricity into light much more effectively than conventional lighting sources. The U.S. Department of Energy has estimated that switching to LED lighting over the next two decades could save the country $250 billion in energy costs over that period, which could reduce the electricity consumption for lighting by nearly one half, and avoid 1,800 million metric tons of carbon dioxide emission.
Low-cost and high-efficiency white light emitting diodes (“WLEDs”) (144 lm/W) are considered a potential light source to replace conventional incandescent (15 lm/W) or fluorescent lighting (114 lm/W) prevalent in major infrastructures. Other applications range from computer displays to car headlights. However, major setbacks of SSL are the cost, the dependence on the rare earth (“RE”) elements, which are in serious supply shortage, and the lack of their recyclability, thus causing damages to the environment. Today, the purchase price of WLED lighting products is significantly higher than that of their conventional counterparts, and the energy savings often are not enough to offset the difference within an attractive payback period.
Common approaches to produce WLEDs include blending of three primary colored LEDs, namely red, green, and blue (“RGB”) diodes, or combination of a blue (or ultraviolet, UV) LED with a yellow phosphor (or multiphosphors). Either process requires complex doping/mixing and delicate control of multiple materials and colors, which proves both challenging and costly. At present, commercially available WLEDs are predominantly phosphor based (e.g., a yellow-emitting phosphor, Ce3+ doped yttrium aluminum garnet or (YAG):Ce3+, coupled with a blue-emitting InGaN/GaN diode). While less expensive than the RGB diodes, the (YAG):Ce3+ type phosphors and WLEDs have issues such as unsuitability for solution process, poor color rendering index (CRI), high correlated color temperature (CCT) and dependence on rare earth (RE) component, which limit their widespread commercialization in general lighting market.
About 20 percent of global RE elements are used in clean energy technologies. Among them lanthanum, cerium, europium, terbium and yttrium are the important components used in the phosphors for energy-efficient lighting. Since the demand for RE elements as an input for different technologies is increasing, their prices have been rising constantly. Comparing to the prices in 2001 the increase level is between four to forty-nine fold in current dollars. Between 2001 and Jul. 19, 2011, the price increases of certain rare earth elements are as follows: lanthanum 3,200%, cerium 2,600%, dysprosium 4,900%, terbium 1,600%, europium 600% and yttrium 400%. (See Critical Materials Strategy, U.S. Department of Energy, available at http://energy.gov/sites/prod/files/DOE_CMS2011_FINAL_Full.pdf, December 2011.)
Global rare earth oxide (REO) production in 2010 was estimated to be 120,000 tones, and it is expected that the total production capacity will increase to 200,000 tons in 2015. However, the demand is much higher than the production, for example, two thirds of the global demand for europium oxide is for use in phosphors. Based on a model calculation, which assumes the limited substitution of LEDs and organic LEDs (OLEDs) for florescent lighting, production capacity has to be increased by 250 tons per year to meet 2025 demand. The estimated supply of yttrium oxide for 2005 is 11,000 tons/year while the demand can be 12,000 tons/year, assuming limited substitution of LEDs or OLEDs for florescent lighting.
The DOE's 2011 Critical Materials Strategy report shows that several rare earth materials that are used in clean energy technologies are at risk of supply disruptions in the short term. Use of clean technologies that are dependent on these materials may be affected by supply challenges that exist for rare earth metals, especially dysprosium, neodymium, terbium, europium and yttrium. Therefore, developing new types of phosphors, especially white/yellow phosphors and/or white/yellow light emitting semiconductor materials that are energy efficient, cost-effective, and free of RE elements is of great importance and in high demand for solid state lighting technologies.